1. Field of the Invention
The present invention relates to a thermochromic color-memory composition. More particularly, the present invention relates to a thermochromic color-memory composition which has remarkable hysteresis characteristics in temperature change to exhibit reversible color change between a colored state and a decolored state, and the colored or decolored state is retained reversibly at ordinary temperature range after stopping the heating or cooling for the color change.
2. Related Background Art
Conventional reversible thermochromic materials change their color with change of temperature at a certain color change temperature. At ordinary temperature, the materials assume only one of the two colors. The other color is retained only when the heating or cooling is being applied to the material to keep the color state. The color changes to the ordinary-temperature color when the heating or cooling is discontinued to bring the material to ordinary temperature.
On the other hand, a thermochromic color-memory material is disclosed in U.S. Pat. No. 4,720,301. This type of thermochromic color-memory material retains any one of the high-temperature color and the low-temperature color selectively at ordinary temperature even after the heating or cooling for the color change is discontinued, and either color may be assumed reversibly at ordinary temperature by heating or cooling, if necessary. Therefore, this type of material is useful in various application fields such as temperature-sensitive recording materials, toys, ornaments, and printing materials.
This kind of color-memory effect could be achieved only with the system employing a specified ester compound for controlling the color reaction. Therefore the usefulness of the thermochromic material was limited, and a more effective compound is eagerly waited for.
It was found by the inventors of the present invention, after comprehensive studies on the color reaction medium for the color-memory thermochromism, that a system employing a compound selected from aliphatic ketones of 10 to 22 total carbons and aryl alkyl ketones of 12 to 24 total carbons has large hysteresis width (.DELTA.H) in the thermochromism to exhibit effective color-memory effect and to give extremely high color contrast between the colored state and the decolored state. Thus the present invention has been accomplished.